Good thing that you don't have that 84 300SD as the vacuum systems on the 85 are completely different. Be real carefull if you use aftermarket info. The systems all changed in 85. One size won't fit all.
I am fairly confused by your vacuum readings and symptoms. Your numbers look like vac in inches not mm. The vacuum shoul be fairly stable once the reservoir is full. The vacuum that you want to monitor for the trans (follow the line to the trans modulator) should have high vacuum at idle and drop with increased throttle to 3-5 inches full throttle. As boost increases this value goes to zero. THIS is one of the reasons the systems are so different in 85. The tranny signal is modified by the big blue amplifier and takes into account not only throttle position but amount of boost.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician